1. Field of the Invention
The present invention relates to the structures of capacitive pressure sensors requiring hermetic sealing.
2. Description of the Related Art
A known capacitive pressure sensor for measuring absolute pressure includes a silicon substrate and a glass substrate that are bonded so as to seal a microgap therebetween, thereby forming a reference pressure chamber. The silicon substrate has a pressure-sensitive diaphragm portion for sensing the pressure to be measured.
An electrode is formed on the pressure-sensitive diaphragm portion by diffusing an impurity using semiconductor manufacturing technology while another electrode is formed on the glass substrate by depositing a thin metal film using, for example, vapor deposition or sputtering. Leads from the two electrodes are connected to the outside via a diffusion lead and a metal lead extending between the bonding surfaces of the silicon and glass substrates. The diffusion lead and metal lead are connected to the corresponding electrodes of a capacitance-sensing IC chip with gold (Au) wires by wedge bonding.
Examples of known structures for sealing the bonding surfaces of the silicon and glass substrates, between which the diffusion lead and metal lead extend, include a sealing structure in which the parts to be sealed are clamped with a resin material such as polyimide (for example, see Japanese Unexamined Patent Application Publication No. 07-209117), a sealing structure in which a metal film is formed locally at a lead portion (for example, see Japanese Unexamined Patent Application Publication No. 08-097438), and a sealing structure in which a vacuum space is created and sealed by forming a silicon oxide film using low pressure chemical vapor deposition (LPCVD) (for example, see Japanese Unexamined Patent Application Publication No. 11-284204).
According to another known structure, a silicon sealing cover is disposed over a through hole at an electrode lead portion (for example, see Japanese Unexamined Patent Application Publication No. 04-009727).
FIG. 10 illustrates an example of a sectional structure of a conventional capacitive pressure sensor 30 disclosed in Japanese Unexamined Patent Application Publication No. 04-009727.
In FIG. 10, a silicon substrate 31 having a pressure-sensitive diaphragm portion 33 for sensing the pressure to be measured is bonded to a glass substrate 32 to form a microgap as a reference pressure chamber 40. A first electrode 35 made of a thin metal film is formed on the side of the glass substrate 32 facing the reference pressure chamber 40 by vapor deposition or sputtering. The glass substrate 32 is perforated so as to form a first hole portion 38. This hole portion 38 is sealed on the reference pressure chamber 40 side by bonding a silicon cover plate 36. A lead portion 37 made of a thin metal film is formed over the surface of the cover plate 36 inside the hole portion 38 and the inner surface of the hole portion 38 by vapor deposition or sputtering. Accordingly, the electrode 35 is electrically connected to the lead portion 37 via the cover plate 36.
On the other hand, a second electrode 34 is formed on the side of the pressure-sensitive diaphragm portion 33 facing the reference pressure chamber 40 by diffusing an impurity onto the overall surface of the silicon substrate 31. A diffusion surface 51 formed on the silicon substrate 31 between the bonding sides of the silicon substrate 31 and the glass substrate 32 is electrically connected to another lead portion formed in a second hole portion (not illustrated) formed in the glass substrate 32.
A capacitance-sensing IC chip 39, which is an interface circuit, is bonded to the glass substrate 32. The first and second lead portions are connected to the corresponding electrodes of the capacitance-sensing IC chip 39 with gold (Au) wires by wedge bonding.
The portion of the silicon substrate 31 that is opposed to the cover plate 36 bonded to the glass substrate 32 is recessed in agreement with the protrusion of the cover plate 36 so that the first electrode 35 of the glass substrate 32 can be electrically insulated from the second electrode 34 of the silicon substrate 31.
The clamping structure using a resin as shown in Japanese Unexamined Patent Application Publication No. 07-209117 and the sealing methods using a metal or oxide film as shown in Japanese Unexamined Patent Application Publication No. 08-097438 and Japanese Unexamined Patent Application Publication No. 11-284204, however, lack long-term reliability and have a problem in terms of resistance to higher pressures. In particular, the sealing structures in Japanese Unexamined Patent Application Publication No. 07-209117 and Japanese Unexamined Patent Application Publication No. 08-097438 also have a problem in that they involve a complicated process including the step of depositing a part of a lead portion and are therefore unsuitable for mass production.
In addition, the sealing structure using a silicon cover plate as disclosed in Japanese Unexamined Patent Application Publication No. 04-009727 further involves the step of forming the cover over a through hole, and also has difficulty in reducing the size of devices because this structure requires a space for attaching the cover plate.